Investigating the impact of gut microbiota recolonization in recovering the muscular defects induced by microbiota depletion in germ-free mice.

The gut microbiota (GM) influences systemic health by communicating with peripheral organs, through the release of microbiota-derived metabolites. Recently, the gut-peripheral nervous system connection was identified, showing that GM depletion in germ-free (GF) mice negatively impacts on peripheral nerves and their target, skeletal muscles. My thesis, rooted in this previous work, explored the effects of GM recolonization in rescuing the muscular alterations described in GF mice. I analysed gastrocnemius muscles from four different groups of mice: i) GF; ii) Oligo-Mouse-Microbiota 12 (OMM12), recolonized with 12 bacterial strains; iii) complex gut microbiota (CGM); iv) ex-germ-free (ex-GF), GF mice recolonized with CGM microbiota after weaning. Morphometric analyses showed that the reduction in muscle weight and fast-twitch fiber size observed in GF and OMM12 mice was restored in ex-GF mice. Similarly, ex-GF mice recovered aberrant molecular features, including upregulation of Trim63 and Fbxo32 atrogenes and reduced Akt and S6 phosphorylation levels, pointing to increased protein degradation and decreased protein synthesis. Elevated LC3-II and reduced p62 levels in GF and OMM12 mice, suggesting enhanced autophagy and proteolysis, were also normalized in ex-GF muscles. These results demonstrate the reversibility of GF-dependent muscle defects through GM restoration, offering insights into GM-based therapeutic strategies targeting muscle-wasting disorders.